The market trends for solid state image sensor technology represents a sales potential exceeding a billion dollars, with device sales many times that figure. Established markets for the technology include space based sensors, computer multimedia devices, video phones, toys, cameras, surveillance equipment, automotive, personal imaging equipment, x-ray imaging, manufacturing inspection, and telemedicine. Initial strides in the solid state image sensor technology resided with the silicon charge-coupled device (CCD) technology. More recently Active Pixel Sensor (APS) array technology is being pursued as an alternative to CCD array technology in the established markets. Unlike CCD technology, APS technology is compatible with standard VLSI (Very Large Scale Integration) fabrication technologies. This compatibility presents opportunities for integrating imaging arrays and vast signal processing functionality on a single silicon chip. APS technology is also free of the fundamental scaling limitations that have prevented CCD technology from producing the very large imaging arrays required for high resolution imaging.
For APS imagers, those skilled in the art recognize the advantages of using arrays of photosensing devices that produce signal output in voltage form rather than in current form. Additionally, the level of the voltage signal produced per unit device area should be high in APS imager applications. If the level of the output signal is low, a voltage amplifier (scaling) circuit must be included at each pixel site, which results in an increase in pixel site area and a lower available signal to noise ratio. More generally, the level of output desired from the photosensing device is a level that matches the dynamic range capabilities of the circuit which subsequently processes the signal. To achieve this match, the prior art devices require boosting the output of the photosensing device with a cascode-type amplifier stage. As is well known, a cascode-type amplifier stage is a single transistor circuit acting as an impedance transformer. However, this approach suffers the disadvantages of added area at each pixel site and an undesirable special bias voltage line running across the array to reach each cascode site. Moreover, the cascode-type amplifier stage used in prior art APS arrays degrades the signal to noise ratio and places a compromising limit on the level of bias that can be applied across the photosensing device.
Accordingly, there is a need for an integrated photosensing device for Active Pixel Sensor (APS) imagers that overcomes the shortcomings of the prior art devices which utilize cascode-type amplifier stages.